1. Field of the Invention
The present invention relates generally to an FM radio for an automotive vehicle, and more particularly to the output circuit of an FM radio in which a bass sound switching unit (a bass sound reducing filter) is provided, by which stereophonic broadcasting can be enjoyed with enough bass sound and monaural broadcasting can be enjoyed without excessive bass sound but with sufficient articulation.
2. Description of the Prior Art
As is well-known, there have been various FM radio sets which have wide reproducing characteristics covering the audible range completely. In these FM radio sets, usually, a loudness control circuit is provided. This is because at lower sound levels, the sensitivity of the human ear in the high and low frequency ranges drops very markedly, and the loudness control circuit is used to adjust the output voltage-frequency response of the amplifier to compensate for this, so that the sound quality does not appear to change when the sound level changes. In other words, the output voltage-frequency response of the amplifier has flat top characteristics when the sound level is comparatively large, but the response must be strengthened in the low and high audio frequency ranges when the sound level is small, so that amplification characteristics may be fitted to human hearing characteristics.
When an FM radio set having a loudness control circuit is receiving a music broadcast with a wide dynamic range, the loudness control will provide good loudness level conditions, but when the FM radio set is receiving a speech broadcast with a narrow dynamic range by using a loudness control circuit, the sound level will be excessively strengthened in the low audio frequency range and therefore articulation will be impaired especially in speech broadcasting.
FM broadcasting includes FM stereophonic broadcasting used mainly for music and FM monaural broadcasting used mainly for conversation. Therefore, the present invention has come from an idea to switch the loudness control circuit in response to FM stereophonic broadcasting or FM monaural broadcasting.